A TV tuner needs to cover a wide reception band allocated to TV broadcast signals. For example, in Japan, a TV tuner needs to support VHF (Very High Frequency) channels (100 MHz band, 200 MHz band) and UHF (Ultra High Frequency) channels (470 MHz to 770 MHz). Also, a software radio needs to support a plurality of radio systems that use different radio bands.
In general, when a radio frequency signal of a frequency band that is an odd multiple of a local oscillation signal for driving a mixer is input to a mixer configuring a radio reception section, a disturbing signal frequency-converted to a frequency in the vicinity of received signal output having a desired frequency is output due to a nonlinear characteristic of the mixer. Also, in general, it is possible to suppress a harmonic response that is an even multiple of a local oscillation signal by giving a mixer a differential configuration, and using differential combining in subsequent-stage circuitry. However, if a radio frequency signal of a frequency band that is an even multiple of a local oscillation signal is input due to a mismatch between differential circuits, a disturbing signal frequency-converted to a frequency in the vicinity of received signal output having a desired frequency is output. Below, this disturbing signal is referred to as harmonic response.
Here, if a reception band that should be supported by a TV tuner or software radio is wide, and the ratio between signal amplitude when a signal of a desired frequency is received and signal amplitude of other than a desired frequency component output due to harmonic response reaches a predetermined value, reception sensitivity degrades. Thus, technology is known that suppresses harmonic response by approximating an output waveform to a sine wave (see Patent Literature 1 through Patent Literature 6, and Non-Patent Literature 1 and Non-Patent Literature 2).
Also, in recent years, with the object of simplifying the circuitry of a radio reception section, a direct conversion reception method whereby the output frequency of a high frequency processing mixer is set in the vicinity of zero hertz, or a Low-IF (Intermediate Frequency) reception method, have become mainstream. With these configurations, a local oscillation signal leaks to an RF input terminal of a mixer, this leakage component is reflected by a preceding-stage circuit and is input to the mixer again, and a DC component is output from the mixer. It is known that reception quality degrades due to fluctuation of the amplitude of this DC component. Below, this phenomenon is referred to as self-mixing.
In general, a reflection coefficient between a Low Noise Amplifier (LNA) connected ahead of a mixer and the mixer is changed by changing the LNA gain setting. Also, a reflection coefficient between an antenna and subsequent-stage circuitry changes according to a change in conditions around the antenna. The amplitude of a leakage component of a local oscillation signal fluctuates due to changes in these reflection coefficients. As a result, the amplitude of a DC (Direct Current) component fluctuates, causing degradation of reception sensitivity (self-mixing). A technology for suppressing this self-mixing is known whereby the ratio of an on-period to one cycle of a mixer driving signal (hereinafter referred to as the duty ratio) is made 25% (see Non-Patent Literature 3).
Also, in recent years there has been a technology called charge sampling, and charge sampling circuit 10 having a configuration such as shown in FIG. 1 is known (see Non-Patent Literature 4, for example). Switch 2 is subjected to on/off control by means of a control signal comprising a rectangular pulse input from control terminal 11, and a current output from current source 1 charges capacitative element 3 only while switch 2 is on.
A filter characteristic due to a current integration effect is obtained according to this charging period. Here, it is known that, among attenuation pole frequencies, a frequency closest to zero hertz changes according to the control signal duty ratio. Specifically, when the duty ratio is made N %, an attenuation pole is generated at a frequency position 100/N times the control signal (local oscillation signal) frequency.